Electron spin resonance spectrometer

ABSTRACT

An electron spin resonance spectrometer comprises a resonator arranged in a magnetic field of constant strength and high homogeneity and containing a sample. The resonator can be supplied, via a microwave bridge, with microwave energy in the form of a continuous-wave signal (CW) or in the form of an intermittent signal (P). Measuring signals emitted by the resonator can be supplied to a detector arrangement and a signal evaluation arrangement. In order to be able to carry out individual experiments with continuous-wave signals or intermittent signals of low power or with intermittent signals of high power in arbitrary combination and with an arbitrary succession of microwave pulses, the spectrometer comprises a first channel for supplying a continuous-wave signal or an intermittent signal of small power in the mW range and a second channel for supplying an intermittent signal of high power in the W range. The channels are connected by their inputs to a common microwave source and united at their outputs. So the said channels can be selectively operated either individually or jointly.

The present invention relates to an electron spin resonance spectrometercomprising a resonator arranged in a magnetic field of constant strengthand high homogeneity and containing a sample, in which the resonator canbe supplied, via a microwave bridge, with microwave energy in the formof a continuous-wave signal or in the form of an intermittent signal andin which measuring signals emitted by the resonator are supplied to adetector arrangement and a signal evaluation arrangement. An electronspin resonance spectrometer of the type described above has been knownfrom the paper by Schweiger, published in US Z Phys. Rev. Lett. 54, page1241 (1985).

The known electron spin resonance (ESR) spectrometer uses asingle-channel arrangement which can be operated alternatively in thecontinuous-wave (CW) mode or in the intermittent mode (so-called pulseoperation). The microwave channel comprises a power amplifier with atravelling-wave tube for generating the high microwave peak powersnecessary for ESR pulse experiments. However, this single-channel designis connected with two disadvantages inherent to the system:

On the one hand, the use of a travelling-wave tube as a power amplifierlimits the pulse duty factor, i.e. the relation between the pulse lengthand the pulse interval, to a relatively small value of, for example, 1%.Accordingly, when travelling-wave tubes are used, only relatively short"pulses" (correctly: bursts) can be produced, and one always has toobserve a minimum interval between two successive pulses in pulsesequences. This is due on the one hand to the switching times obtainedwith travelling-wave amplifiers and, on the other hand, to the fact thatwhen carrying out pulse experiments one normally wishes to be free toset the phase of successive pulses at desire, and the phaseswitchingelements normally used also require a certain switching time.

The second essential disadvantage of the known singlechannel design liesin the fact that it is unsuited for continuous-wave experiments with lowmicrowave power because in this latter case the inherent noise of thetravellingwave tube would make itself felt as very disturbing. Due tothese drawbacks, the application of the known arrangement is restrictedin numerous ways.

Now, it is the object of the present invention to improve a spectrometerof the type described above in such a manner that it permits theselective running of continuous-wave experiments, pulse experiments orof combinations of these types of experiments, with pulse sequences inwhich individual pulses may follow each other even without any deadtime, in the extreme case.

This object is achieved by the solution provided by the inventionaccording to which the spectrometer comprises a first channel forsupplying a continuous-wave signal or an intermittent signal of smallpower in the mW range and a second channel for supplying an intermittentsignal of high power in the W range, and in which the channels areconnected by their inputs to a common microwave source and united attheir outputs so that the channels can be selectively operated eitherindividually or jointly.

This solves the object underlying the invention fully and perfectlybecause now two absolutely independent channels are available for smallpower on the one hand and high power on the other hand, with each ofthese channels contributing its particular advantages to the experimentto be carried out, while their inherent drawbacks are limited to theirrespective very own application so that they cannot disturb theexperiment.

So, it is possible for example to carry out usual continuous-waveexperiments with low microwave power and high signal-to-noise ratiousing the spectrometer according to the invention, with the secondchannel switched off, without such experiments being disturbed by thenoise of the travelling-wave tube as the latter is a part of the second,switched-off channel. Also, it is possible to carry out experiments withintermittent signals of low power (with so-called "soft pulses"), andthis either with or without an additional basic continuous-wave signal.

If on the other hand, in the reverse case, the first channel is switchedoff and the second channel is switched on, pure pulse experiments can becarried out with high microwave power, and the inherent noise of thetravelling-wave tube does not make itself felt in a disturbing mannerbecause it is not noticeable due to the high microwave power employed inthese experiments.

Finally, when both channels are switched on, it is also possible tocarry out experiments where the sample is subjected simultaneously tomicrowave energy of low power, in continuous-wave or pulse operation,and additionally to microwave energy of high power in pulse operation.In this case, the microwave pulses of small and high power are setindependently of each other as regards their duty cycle, length andphase, so that the two pulses may be spaced relative to each other atdesire and may be irradiated either independently of each other in time,or in immediate succession, without any dead time between the pulses, orso that they overlap each other in time fully or partly, in definedrelationship.

In addition, it is possible in this or in any other of thebefore-described cases where microwave pulses of low power are used, toselect pulse programs with arbitrary pulse duty factors because there donot exist any limitations regarding the pulse duty factor for thelow-power microwave channels. The following are two typical applicationsfor such combined experiments:

On the one hand, a short microwave pulse of high power may be used toachieve wide-band excitation of a spin package, while on the other handelectric saturation of a very narrow range of the spin package can beachieved by a microwave pulse of small power but long duration because,as is generally known, the spectral width of the frequencies produced isinversely proportional to the pulse length.

Another application consists in so-called "saturation recovery"experiments in which initially spins of one measuring sample aresaturated by microwave pulses of high power and the physical recovery ofthe saturation is then observed with the aid of a continuous-wave signalof low power (so-called "observation power").

In order to achieve all these advantages, the first channel of thespectrometer is preferably provided with means for setting the amplitudeand phase of the microwave signal, and the first signal comprises amicrowave switch that can be switched at selected times by means of acontrol input. Preferably, the second channel is further equipped with atravelling-wave amplifier in order to make available the high microwavepowers.

According to a preferred improvement of the invention, the secondchannel is provided with a pulse-shaping stage having a plurality ofparallel pulse-shaping channels, each of them being provided withindividually controllable means for setting the amplitude and phase andfor switching the microwave signal.

This feature provides the advantage that it offers the possibility toselect a pulse sequence for the second signal where the individualpulses can be adjusted absolutely independently of each other, asregards their point in time, magnitude and phase. Accordingly, no deadtimes are encountered between the pulses due to switching times of themicrowave switches or switching times of the phase switches since thepulses of several parallel pulse-shaping channels are combined to apulse, sequence, independently of each other.

Other advantages of the invention will become apparent from thefollowing specification and the drawing. It goes without saying that thefeatures that have been described above and will be explained hereaftercan be used not only in the described combinations, but also in anyother combination or individually, without leaving the scope of thepresent invention.

Certain embodiments of the invention will be described hereafter ingreater detail with reference to the drawing in which:

FIG. 1 shows a largely simplified block diagram of an electron spinresonance pulse spectrometer;

FIG. 2 shows a simplified block diagram of the microwave components ofthe spectrometer according to FIG. 1;

FIG. 3 shows the development in time of two microwave pulses,illustrating the parallel application of microwave circuits of low andhigh power;

FIG. 4 shows the spectral distribution of microwave energy when thetechnique illustrated in FIG. 3 is applied;

FIG. 5 shows another block diagram illustrating details of a high-powermicrowave channel of the block diagram shown in FIG. 2;

FIG. 6 shows a block diagram of other details of pulseshaping channelsof the block diagram shown in FIG. 5;

FIG. 7 illustrates the dependence of time of microwave pulses of thetype that can be produced with the aid of a pulse-shaping channelaccording to FIG. 6;

FIG. 8 shows a block diagram of a variant of the embodiment shown inFIG. 6;

FIG. 9 illustrates the dependence of time of successive microwave pulsesof the type which can be used for synthesizing pre-determineddistribution curves and which can be generated with the aid of thepulse-shaping channels of FIGS. 6 and 8;

FIG. 10 shows a block diagram illustrating other details of a switchingstage as shown in FIG. 5; and

FIG. 11 shows a block diagram illustrating the signal evaluatingcircuits of the electron spin resonance pulse spectrometer of FIG. 1.

In FIG. 1, reference numeral 10 indicates, generally, an electron spinresonance pulse spectrometer of conventional basic configuration.Between the poles of an electromagnet 11 of high homogeneity, there isarranged a microwave resonator 12 within which the sample 13 to beexamined is exposed to the magnetic field components of the microwavefield and, simultaneously, to the constant magnetic field of theelectromagnet 11. A microwave bridge 14 is connected to the resonator 12via a microwave line 15 for the purpose of transmitting excitingmicrowave energy to the resonator 12 and, on the other hand, forreceiving and processing the signals reflected by the resonator 12.Alternatively, processing of the signals may be effected by aconventional detector 16 connected to a signal amplifier 17. The signalamplifier 17 and numerous other components of the spectrometer 10 areconnected to a central computer control unit 18 via a data line. On theother hand, the signal may be evaluated also via a quadrature detector19 connected to an analog-to-digital converter with sample-and-holdstage, which in turn is connected again to the computer control unit 18via a data line.

In order to enable the spectrometer 10 to be operated in the microwavepulse mode, a pulse program control 25 is provided which is connected tothe computer control unit 18 via a data line and which controls themicrowave bridge 14, the analog-to-digital converter 20 and apulse-shaping stage 26 arranged between the microwave bridge 14 and atravelling-wave amplifier 27. The travelling-wave amplifier 27 alsoreceives control signals from the pulse program control 25. The outputof the travelling-wave amplifier 27 in turn feeds the microwave bridge14 for transmitting microwave high-power energy to the resonator 12.

In addition, the spectrometer 10 is provided in the usual manner with afield rheostat 30 controlling a magnet power pack 31 of theelectromagnet 11. A field/frequency lock 32 receives a field-dependentsignal and a signal dependent on the microwave frequency, which are usedby it to control the field rheostat 30 in the conventional manner. Itis, thus, possible to supply microwave pulses to the resonator 12 and/orthe sample 13 arranged therein, by suitable adjustment of the pulseprogram control 25. In this context, the term "pulses" is used todescribe a wavetrain limited in time, i.e. a burst, which can beadjusted as regards the moment of its inset, its phase and amplitude.For detecting the signals received, auxiliary signals are required in acorresponding manner from the pulse program control 25 to enable thesignals received to be detected and evaluated in synchronism with thehigh-power microwave pulses.

FIG. 2 shows the essential microwave components of the spectrometer 10.

A klystron 40 or another suitable microwave source is connected, via anelectronically adjustable attenuator 41, to a first divider 42 whichdivides the microwave energy into two channels. The first output of thefirst divider 42 is connected to a first coupling capacitor 43 whosecoupling output leads to a level control 44 which in turn controls theattenuator 41. Two input signals for continuous-wave operation (CW) andpulse operation (P) enable two steps of the attenuator 41 to be set. Theillustrated arrangement permits the level of the klystron 40 to becontrolled; in a typical application, a klystron having a maximum poweroutput of 1.2 Watts in the X-band is employed whose power output is setto 200 mW in continuous-wave operation (CW) and to 800 MW in pulseoperation (P).

The other output of the first divider 42 leads to a second couplingcapacitor 45 whose coupling output is connected to a frequencycontroller 46 (ASC). The frequency controller 46 coacts with an externalresonator 47 of high quality and influences by its output a power pack48 of the klystron 40. The use of an external resonator 47 of highquality for controlling the frequency of the klystron 40 is advisablebecause resonators 12 of low quality, for example dielectric resonators,are required for electron spin resonance pulse experiments since a largeband width of the resonator 12 is required for pulse operation. Giventhe very low quality of the measuring resonator 12, the latter is notsuited for frequency regulation, for example for compensating possibletemperature drift phenomena. One therefore uses the external resonator47 having a quality of several thousands.

The output of the second coupling capacitor 49 leads to a third couplingcapacitor 49 whose coupling output is connected to a reference branch 50which will be described in greater detail further below, in connectionwith FIG. 11.

The output of the third coupling capacitor 49 leads to a fourth couplingcapacitor 51 whose coupling output is connected to the field/frequencylock 32.

Regarding now the other side of the microwave circuit, the output of thefirst coupling capacitor 43 is connected to a fifth coupling capacitor52 whose coupling output leads to a second divider 53. The seconddivider 53 feeds two monitors 54 and 55 for continuous-wave operation(CW) and pulse operation (P). The monitors 54, 55 are provided withsecond inputs which are supplied with microwave signals received fromthe signal branch and the high-power microwave branch, respectively. Themonitors 54, 55 serve to check the correct setting of the microwavecomponents, in particular the setting of the microwave pulses.

The coupling output of a sixth coupling capacitor 56 connected to theoutput of the fifth coupling capacitor 52 leads to a microwave counter57. It is ensured in this manner that the microwave frequency adjustedat any time is indicated continuously. After various signals have beencoupled out for measuring and control purposes the manner justdescribed, via the coupling capacitors 43, 45, 49, 51, 52 and 56, thetwo microwave channels defined by the first divider 42 lead to acontinuous-wave channel 60 in the upper half of FIG. 2 and a pulsechannel 61 in the lower half of FIG. 2, respectively. The outputs of thechannels 60, 61 are united again in a first combiner 62 and supplied toa first circulator 63 of the microwave bridge 14 whose first subsequentoutput leads to the resonator 12, while its second subsequent outputleads to a signal processing unit indicated at 64 in FIG. 2, which willbe described further below in connection with FIG. 11.

The continuous-wave channel 60 comprises substantially a seriesconnection of a first attenuator 70, a first phase shifter 71 and afirst switching diode (PIN diode) 72 that can be switched via a controlinput 73.

In contrast, the pulse channel 61 - which will be described in greaterdetail further below in connection with FIG. 5 - comprises substantiallya series connection of a pulse-shaping stage 74 with control inputs 75and 76, a second phase shifter 77 and a travelling-wave amplifier 27.

Due to the parallel arrangement of the continuous-wave channel 60 andthe pulse channel 61, and the fact that their output ends are united inthe first combiner 62, it is now possible to operate the systemalternatively in continuouswave operation or in pulse operation; or elseboth operating modes may be set simultaneously for carrying out certainexperiments.

In the typical application, the continuous-wave channel is sized to becapable of supplying a power output of approximately 50 mW to theresonator 12. The first switching diode 72, which typically has a risetime of 1 ns, provides the possibility either to operate the system inthe continuous-wave mode (when the first switching diode 72 is in theopen position) or to supply microwave pulses of low output power to theresonator 12, by actuating the first switching diode 72.

These so-called "soft pulses" may have a considerably greater pulselength than the high-power pulses received from the pulse channel 61,the length in time of the latter being determined by the admissiblemaximum pulse duty factor of the travelling-wave amplifier 27 which isnormally in the range of 1% for the usual amplifiers of this type. Bysetting the first phase shifter 21 in the continuous-wave channel 60 ina suitable manner, the soft pulses may be adjusted to any phase positionbetween 0° and 360° relative to the high-power pulses of the pulsechannel 61.

The arrangement described above allows on the one hand to operate thesystem in the usual low-power continuous-wave mode, by switching off thepulse channel 61 and holding the first switching diode 62 permanentlyopen.

In a second operating mode, the system can be operated exclusively withlow-power microwave pulses, by switching off the pulse channel 61 andactuating the first switching diode 72.

In a third operating mode, the system can be operated in the high-powerpulse mode, by switching off the continuous-wave channel 60 andactuating the pulse-shaping stage 74.

In a fourth operating mode, both channels 60 and 61 may be switched onin parallel, in which case the continuous-wave channel 60 supplieslow-power pulses while the pulse channel 61 supplied high-power pulses.An example of this latter operating mode is illustrated in FIG. 3 wherea first pulse 80 of low power but great length is followed, as afunction of time, by a second pulse 81 of high power but little length.

It is generally known that, due to the dependence of the time domain onthe frequency domain, and as a result of the Fourier transform, thepulse width is inversely proportional to the spectral width of thefrequency distribution in the frequency range. Consequently, a shortpulse in the time domain leads to a very broad frequency distribution,while a long pulse leads to a very narrow frequency distribution. Thisphenomenon can be utilized for the experiment according to FIG. 3 sothat one obtains a frequency distribution as shown in FIG. 4, where arelatively broad frequency distribution 82 caused by the second pulse 81exhibits a very narrow incision originating from the first pulse 80.Accordingly, it is possible by means of the soft pulse 80 to achieveselective saturation (incision 83) in an otherwise broad spin package(frequency distribution 82).

It goes without saying that the representation of FIG. 3, with thepulses 80 and 81, is to be understood as an example only; the pulses mayof course have any desired time relationship, phase position oramplitude relationship, or may, for example, even coincide. Finally, afifth operating mode is possible where the pulse channel 61 supplies thehigh-power pulses described before, while the continuous-wave channel 60operates in the continuous-wave mode because the first switching diode72 is held permanently open. This admixture of a continuous-wave levelof the high-power pulses may be employed to produce the so-called"saturation recovery" which comprises the steps of saturating the spinsystem and detecting thereafter the recovery of the system from thesaturated condition by means of the continuous-wave level.

In this case, the parallel use of a separate low-power continuous-wavechannel 60 provides the advantage that very low continuous-wave levelscan be used without the noise of the travelling-wave amplifier makingitself felt in a disturbing manner, which would be the case if only asingle high-capacity channel were used and the latter were to be changedover to low-level continuous-wave operation after irradiation of thehigh-power pulses.

FIG. 5 shows certain other details of the pulse channel 61.

At the input end of the pulse channel 61, one can see a third divider 90which acts to divide the incoming microwave energy into four paralleland equivalent pulse-shaping channels 91a, 91b, 91c and 91d which at theoutput end are united again by a second combiner 94 which is symmetricalto the third divider 90. Each of the pulse-shaping channels 91a to 91dcomprises two control inputs 92a to 92d or 93a to 93d, respectively,which will be explained in greater detail further below in connectionwith FIGS. 6 and 8.

The output of the second combiner 94 is followed by a series connectioncomprising a preliminary microwave amplifier 95, the second phaseshifter 77, a second switching diode 96 with control input 97, a seventhcoupling capacitor 98 whose coupling output leads to the pulse monitor55, the travelling-wave amplifier 27, a second attenuator 99 and aswitching stage 100 with control input 101.

The four parallel pulse-shaping channels 91a to 9d enable arbitrarypulse programs to be compiled. For example, it is possible to set aso-called X pulse (0° phase) in the first pulse-shaping channel 91a, aso-called -X pulse (+90° phase) in the second pulse-shaping channel 91b,a so-called Y pulse (+180° phase) in the third pulse-shaping channeland, finally, a so-called -Y pulse (+270° phase) in the fourthpulse-shaping channel 91d.

The parallel arrangement of four pulse-shaping channels 91a to 91dprovides the advantage that the before-mentioned X, -X, Y and -Y pulsescan be set to any phase position relative to each other. This would notbe possible within one and the same channel because the usual componentsused for switching the phase of microwave signals exhibit a switchingtime considerably longer than 3 ns so that very close phase relationscannot be realized in this manner. In contrast, the use of parallelchannel makes it possible to set very close phase positions, includingthe phase position 0.

As will be described further below in connection with FIGS. 6 and 8, thepulse-shaping channels 91a to 91d are already provided with switchingdiodes for attenuating the microwave power. The second switching diode96 with control input 97, following the common output of the secondcombiner 94, serves in addition the purpose to enlarge the dynamic rangebecause the travelling-wave amplifier 27 exhibits, for example, adynamic range of 53 dB which cannot be spanned by a single switchingdiode. For attenuating and, thus, utilizing the whole dynamic range ofthe travelling-wave amplifier 27, one therefore uses two such switchingdiodes connected in series, because usual switching diodes exhibit adynamic range of approximately 25 dB.

The second attenuator 99, which may be designed as a highpowerattenuated with a range of, say, 0 to 60 dB, is arranged in the outputof the travelling-wave amplifier 27.

As is generally known, it is a particularity of usual travelling-waveamplifiers that when a microwave pulse is amplified, a so-called "tail",i.e. a decay process, appears at the end of the trailing edge. Thisphenomenon makes itself felt in a very disturbing manner in electronspin resonance pulse experiments. One therefore provides the switchingstage 100 which serves the function to cut off this pulse "tail". Thisreduces the dead time of the spectrometer considerably. The details ofthe switching stage 100 are illustrated in FIG. 10 and described in therelevant part of the specification.

FIG. 6 shows a first variant of an embodiment of a pulse-shaping channel91a.

As can be clearly seen in FIG. 6, here again two parallel branches areprovided which are formed by a fourth divider 110 arranged in the input.Each branch comprises a series connection of a third attenuator 111a or111a, a third phase shifter 112a or 112b, a third switching diode 113aor 113b associated with the before-mentioned control inputs 92a and 93a,and the branches are finally united again in a third combiner 114.

The upper branch 111a . . . in FIG. 6 serves as a so-called "master"branch and generates the desired pulse, for example an X pulse byoperation of the third switching diode 113a when the control input 92ais triggered.

Due to the fact that in spite of its relatively low quality themeasuring resonator 12 still exhibits a limited band width only, a decayprocess described as "ringing" occurs when a high-power microwave pulseis supplied to the resonator. In order to eliminate this disturbingeffect, the lower branch 111b . . . in FIG. 6 is intended to serve as a"slave" branch in which a suppression pulse shifted by 180° relative tothe "master" pulse is generated. A pulse sequence of this type is shownin FIG. 7. Reference numeral 117 designates the so-called "physicalpulse" serving to carry out the electron spin resonance experimentproper, i.e. for exciting the sample 13. The "physical pulse" 17 whichexhibits, for example, the 0° phase (X pulse) is followed by a so-called"technical pulse" 118 exhibiting a phase position of 180° forsuppressing the decay process of the resonator 12 (cavity-ringingquenching pulse).

It is apparent that for reasons of high-frequency technology, thedistance between the pulses 117, 118 should possibly be equal to 0. Forpractical reasons it may, however, be advantageous to provide a smalltime interval between the pulses 117, 118, which is achieved by means ofthe monitors 54 and 55.

Preferably, the elements of the "master" branch 111a . . . and of the"slave" branch 111b . . . are designed identically. As can be seen inFIG. 6, both the phase and the amplitude of the pulse (or of the pulsesequence) can be adjusted separately in each branch. Typically, one willuse an amplitude adjusting range of 0 to 30 dB and a phase adjustingrange of 0 to 360°, with a resolution of 0.1°.

If one regards FIG. 6 in conjunction with FIG. 5, it appears that thefour double-branched pulse-shaping channels 91a to 91d provide a totalof eight, for example identical, microwave channels. This permits thepulse channel 61 to be operated in different operating modes:

In one operating mode, arbitrarily selected pulse sequences can beadjusted by means of the four pulse-shaping channels 91a to 91d, and thedecay process of the resonator 12 can be suppressed in each case. It ispossible in this manner to subject the sample 13 to pulses or pulsesequences of the type known from the magnetic resonance technique. Thepulse sequences of Carr-Purcell or Carr-Purcell-Meiboom-Gill may serveas examples in this connection. Even experiments with so-called phaserotation are possible. The technical differences in the two branches ofthe quadrature detectors 19, which will be described in greater detailfurther below in connection with FIG. 11, and in the associated videoamplifiers can be averaged out by exchanging the phases cyclically andsorting the resulting data in a corresponding manner.

If one designates a measured absorbtion signal by A, and a measureddispersion signal by D, and if X and Y pulses are employed and thephases are cyclically exchanged , one obtains at the output of thedouble video amplifiers at first signals A, D, then signals D, A, thensignals -A, -D and finally signals -D, -A. By adding, subtracting ordividing these signals in a suitable manner it is then possible toeliminate any constant errors.

In this connection, it is of particular advantage that the two branchesin each pulse-shaping channel 91a to 91d allow extremely short deadtimes because each physical pulse (117 in FIG. 7) is followed by atechnical quenching pulse (118 in FIG. 7).

The following is another operating mode which is rendered possible bythe totally eigth microwave channels:

In cases where it is desired to subject the sample 13 to very selective,i.e. narrow-band excitation, the shape of the microwave pulse shouldassume the form of a Gaussian curve in the time domain. Such adistribution curve can be synthesized easily with the aid of eightmicrowave channels, by causing all the eight channels to supplysimultaneously a specific amplitude each. Considering that each of theeight channels can be adjusted individually as regards its amplitude andthat all the eight channels are mixed at the output by the second andthird combiners 94, 114, it is possible in this way to achieve anamplitude resolution of 2⁸, i.e. of 1:256.

FIG. 8 shows the result of this experiment carried out with a pulseshape 120 of Gaussian distribution composed of individual pulses 121,which are generated in succession. As mentioned before, the amplituderesolution 122 achievable is 1:256.

FIG. 9 shows a variant of a pulse-shaping channel 91a' which differsfrom the embodiment of FIG. 6 by the fact that circulators 115 or 115awhich have their third outputs connected each to a waveguide termination116 or 116a, respectively, are provided between the elements 110, 111a,112a, 113a, 114 and 110, 111b, 112b, 113b, 114 shown in FIG. 6. Thecirculators 115 used at the output of the fourth divider 110 and at theinput of the third combiner 114 may consist of duplex insulators with abackward attenuation of 40 dB, while the remaining circulators mayconsist of single insulators with a backward attenuation of 25 dB.

By providing the additional circulators 115 and 115a, the absence ofinteraction in the system is improved and the before-mentioned eightchannels are decoupled from each other.

FIG. 10 shows additional details of the switching stage 100 representedin FIG. 5 at the output of pulse channel 61.

It is of course possible to use a simple switching diode for switchingand/or attenuating the microwave power, which is in fact the case withthe second switching diode 96 in the pulse channel 61. However, wherelarge microwave powers have to be switched, this may lead to problemsbecause the very high microwave power applied may cause the diode to beconnected through automatically, due to an avalanche effect.

Problems of this type can be avoided by the use of a switching stage 100as represented in FIG. 10. Instead of connecting a switching diode intothe microwave branch, a third circulator 130 is employed whose secondconnection is connected to a fourth switching diode 31 which in turnleads to a second waveguide termination 132. The fourth switching diode131 may be switched to a conductive and a nonconductive state via thecontrol input 101.

As can be further seen in FIG. 10, the elements 130, 131 and 132 may beprovided also in cascade arrangement, by connecting a desired number ofadditional element sets 130a, 131a, 132a in series.

As mentioned already with reference to FIG. 5, the switching stage 10serves the purpose to cut off the high-power output pulse of thetravelling-wave amplifier 27 in order to prevent this pulse "tail" fromdisturbing the electron spin resonance experiments. This is achieved bytriggering the control input 101 in time-synchronism with, or with aslight delay after the trailing edge of the microwave pulse. The fourthswitching diode 131 is initially in the blocked condition so that thehigh-power microwave pulse can be guided across the circulator 130because the blocked switching diode 131 reflects the microwave pulsewithout getting itself in the avalanche area. It has of course to betaken into consideration in this connection that the insertion loss ofthe fourth switching diode 131 must be sized suitably to ensure that theswitching diode is not switched to the conductive state when reflectingthe high-power microwave pulse.

The trailing edge of the microwave pulse now sets the fourth switchingdiode 131 to the conductive state so that the pulse "tail" is guided inthe circulator 130 via the fourth switching diode 131 into the waveguidetermination 132. If switching-through of the fourth switching diode 131is delayed a little relative to the trailing edge of the microwavepulse, one may take advantage of the fact that the output pulse of thetravelling-wave amplifier 27 has lost approx. 30 to 40 dB already aftera very short period of time so that the fourth switching diode 131 hasto switch through only the level remaining after this loss of 30 to 40dB. This switching behavior will be sufficient for most electron spinresonance experiments because the steepness of the trailing edge issufficiently great during the short period of time between the trailingedge of the microwave pulse and switching-on of the fourth switchingdiode 131.

This cutting of the pulse "tail" may of course be improved further bythe cascade arrangement of the elements indicated in FIG. 10.

FIG. 11 finally shows additional details of the signalprocessingarrangement.

The measuring signal reflected by the resonator 12, which contains theinformation on the electron spin resonance experiment to be recorded,can be picked up in the usual manner at the third connection of thecirculator 63. It is supplied initially to an eighth coupling capacitor140 from which a coupling output leads to the monitor 54 of thecontinuous-wave channel. The output of the eighth coupling capacitor 114is initially connected to one input of a first change-over switch 141whose second input is available for connection of another resonator, forexample an induction resonator.

The output of the first change-over switch 141 is connected to a fifthswitching diode 143 with control input 144 which in turn leads to asecond change-over switch 145. In the one position of the secondchange-over switc 145, the measuring signal is passed on unchanged,while in another switch position a preliminary microwave amplifier 146is switched on which in a typical application exhibits an amplificationof 38 dB, with a noise factor of only 1.9 dB. In order to protect thepreliminary microwave amplifier 146 in this operating mode fromhigh-power pulses, the fifth switching diode 143 is provided which is inthe open state only during the pulse intervals, in order to supply thefree induction loss of the electron spin resonance signal to thepreliminary microwave amplifier 146 during the pulse intervals, whileduring the rest of the time it protects the latter against thehigh-power pulses.

A third change-over switch 147 unites the before-described branchesagain and feeds a fourth change-over switch 148 whose upper switchingposition--viewed in FIG. 11--is used for continuous-wave measurements,while its lower switching position is preferably used for pulsemeasurements. In the upper switching position of the fourth change-overswitch 148, a continuous-wave channel 150 comprising a fourth combiner151, a single-phase detector 52, for example a coaxial Schottky-Barrierdiode, is activated which amplifies the total signal formed from themeasuring signal (arriving from the fourth change-over switch 148) andthe reference signal (supplied via the second input of the fourthcombiner 151) with a band width of, for example, 30 Hz to 5 MHz, andprocesses it thereafter in the usual manner. Finally, the electron spinresonance signal is displayed by an indication unit 153, for example adisplay, a printer, or the like. In addition, it may be stored,evaluated or processed in any other manner as known in the art. In thelower switching position of the fourth change-over switch 148, themeasuring channel shown in the lower half of FIG. 11 and comprising thereference branch 50 and a subsequent quadrature channel 160, isactivated.

In the reference branch 50, the microwave reference signal coupled outby the third coupling capacitor 49 reaches at first the seriesconnection composed of a delay element 161, a fourth attenuator 162 anda fourth phase shifter 163, before it is split up in a fifth divider164. A first output of the fifth divider 164 leads to the input of asixth divider 165 whose one output supplies the reference signal for thefourth combiner 151 which has been described above in connection withthe continuous-wave channel 150. The second output of the fifth divider164 leads to the quadrature channel 160, via the series connectionconsisting of a fifth phase shifter 166 and a fifth attenuator 167.

A sixth divider 168 provided in this quadrature channel 160, on itsinput end, has its input connected to the fourth change-over switch 148.The two outputs of the sixth divider 168 lead to a first quadraturedetector 169 and a second quadrature detector 170, respectively,designated in FIG. 1 by the common reference numeral 19. The secondinput of the first quadrature detector 169 is connected to the secondoutput of the sixth divider 165, while the second input of the secondquadrature detector 170 is connected to the output of the fifthattenuator 167. The outputs of the quadrature detectors 169, 170 arefinally connected to the analogue-to-digital converter 20 withsample-and-hold stage, and this unit is in turn connected to the displayunit 153, via additional elements of a conventional type which are notshown in detail in FIG. 11. During pulse operation, the so-called"FID"--free induction decay--is measured in two orthogonal projectionsof the circulating magnetization.

As results from the above, the quadrature detectors 169 and 170 aresupplied on the one hand--via the sixth divider 168 --with the measuringsignal and, on the other hand, with a reference signal, which latter canbe adjusted in the second quadrature detector 170 by means of theelements 166, 167, as regards its amplitude and magnitude relative tothe other reference signal at the lower output of the sixth divider 165.

For normal spin echo experiments it will of course be sufficient to useonly one quadrature detector 169 or 170, in which case the other branchis switched off. The quadrature detectors 169, 170 used consist ofmicrowave mixers with a high dynamic ratio.

The block 20 shown in FIG. 11 contains firstly a two-channel videoamplifier having a band width of 50 MHz to 200 MHz and an amplificationof, for example, 66 dB. The output signals of this video amplifier arethen supplied to an analogue-to-digital converter and from there to thecomputer control unit 18. For the sake of clarity, all these details arenot shown separately in FIG. 11.

Generally, commercially available microwave and highfrequency componentsof the type usual and known in electron spin resonance spectrometry maybe employed for the described components of the spectrometer 10.

We claim:
 1. An electron spin resonance spectrometer comprising:anelectromagnet for generating a magnetic field of constant strength andhigh homogeneity over a predetermined space; a microwave resonatorarranged in said space and receiving a sample under investigation; amicrowave generator for generating a continuous wave microwave signaland for supplying microwave energy to a microwave bridge, said microwavebridge being connected to said resonator to supply microwave energy tosaid resonator and being, further, connected to signal detection meansfor receiving and processing signals emitted from said resonator; amicrowave divider having a first and a second output for dividing saidmicrowave continuous wave signal from said microwave generator into twoportions; a first low-power continuous wave channel connected to saidfirst output of said microwave divider and comprising means foradjusting the phase and amplitude one of said microwave signal portionsto provide a first output continuous wave signal in the Milliwatt-range;a second high-power pulse channel connected to said second output andcomprising means for switching and amplifying and for adjusting phaseand amplitude of the other of said microwave signal portions to providea second an output pulse signal being in the Wattrange; a microwavecombiner for combining said first and second output signals and forfeeding said combined signals to said microwave bridge; means foroperating said low-power continuous wave channel and said high-powerpulse channel either separately or jointly.
 2. A spectrometer accordingto claim 1, wherein the said first channel comprises means for adjustingthe amplitude and the phase of the microwave signal.
 3. A spectrometeraccording to claim 1, wherein the said first channel comprises amicrowave switch which can be switched at selective times by means of acontrol input.
 4. A spectrometer according to claim 1, wherein the saidsecond channel comprises a travelling-wave amplifier.
 5. A spectrometeraccording to claim 1, wherein the said second channel is provided with apulse forming stage having a plurality of parallel pulse formingchannels, each of them being provided with individually controllablemeans for setting the amplitude and phase and for switching themicrowave signal.